1. Field of the Invention
The present invention relates to a high corrosion resistant plated composite steel strip and a method for producing the same. More particularly, the present invention relates to a high corrosion resistant plated composite steel strip having a composite coating layer comprising a zinc-based electroplating layer containing corrosion resistance-promoting solid particles, and a method for producing the same.
2. Description of the Related Art
It is known that, in the winter in North America and Europe, freezing (icing) of road surfaces is prevented by sprinkling rock salt powder or calcium chloride powder on the road surface. However, the above mentioned icing-preventing material causes corrosion and rusting of the bodies of cars traveling on those roads.
Accordingly, there is a demand for a high corrosion resistant plated steel strip for car bodies which can be used under the above-mentioned circumstances, without allowing the forming of red rust on the car bodies, over a long period.
There are two approaches for meeting the above-mentioned demand.
In countries, for example, the U.S.A. and Canada, where the cost of electricity is relatively low, the corrosion resistance of the steel strip is promoted by forming a thick corrosion resistant coating layer on the steel strip. However, the thick coating layer causes the resultant coated steel strip to exhibit a reduced weldability, paint adhesion and plating propeties.
In other countries, for example, Japan, where electricity is expensive and a high weldability and good paint adhesion and plating properites are required for the steel strip to be used for car bodies, a plated steel strip having a thin corrosion resistant electroplating layer has been developed.
The plated steel strip of the present invention belongs to the above-mentioned category of plated steel strips having a thin corrosion resistant electroplating layer.
In this type of conventional electroplated steel strip having a thin electroplating layer, a zinc alloy, for example a zinc-iron, zinc-nickel or zinc-manganese alloy, is plated on a steel strip substrate, or zinc or a zinc-nickel alloy is electroplated on a steel strip substrate and a chromate treatment and an organic resinous paint are then applied to the electroplating layer. The zinc alloy-electroplated or zinc or zinc alloy-electroplated and painted steel strips have a thin coating layer at a weight of 20-30 g/m.sup.2. The conventional electroplated steel strips having the above-mentioned thin coating layer are not considered satisfactory for attaining the object of the domestic and foreign car manufacturers, i.e., that the car bodies should exhibit a resistance to corrosion to an extent such that rust does not form on the outer surfaces of the car bodies over a period of use of at least 5 years, and perforation from the outer and inner surfaces of the car bodies does not occur over a period of use of at least 10 years. In particular, a 10 year resistance to perforation is demanded.
Under the above-mentioned circumstances, recently, investigations have been made into the obtaining of a high corrosion resistant steel strip having a coating layer in which corrosion resistive fine solid particles are co-deposited within a plating metal matrix and are evenly dispersed within the plating metal matrix, i.e., a high corrosion resistant plated composite steel strip.
The co-deposited, dispersed fine solid particles can impart various properties to the plating layer of the plated composite steel strip, and thus this co-deposition type plating method has been developed as a new functional plating method. Namely, this type of plating method has been recently disclosed in Japanese Unexamined Patent Publication Nos. 60-96786, 60-211094, 60-211095 and 60-2111096.
Japanese Unexamined Patent Publication No. 60-96786 discloses a method for producing a plated composite steel strip in which fine solid particles of rust-resistant pigments, for example, PbCrO.sub.4, SrCrO.sub.4, ZnCrO.sub.4, BaCrO.sub.4, Zn.sub.3 (PO.sub.4).sub.2 are co-deposited with a plating metal matrix, for example, Zn or a Zn-Ni alloy, to be evenly dispersed in the plating metal matrix. This type of plated composite steel strip is considered to have an enhanced resistance to rust and perforation. However, according to the result of study by the inventors of the present invention, the plated composite steel strip of Japanese Unexamined Patent Publication No. 60-96786, in which the fine solid particles dispersed in the plating layer consist of rust-resistant pigments consisting of substantially water-insoluble chromates, for example, PbCrO.sub.4, SrCrO.sub.4, ZnCrO.sub.4 or BaCrO.sub.4, cannot realize the above-mentioned corrosion resistance level of no rust for at least 5 years and no perforation for at least 10 years. This will be explained in detail hereinafter.
Generally, the rust resistant pigment fine particles of the substantially water-insoluble chromates dispersed in a zinc-plating liquid exhibit a surface potential of approximately zero, and accordingly, when a steel strip is placed as a cathode in the zinc-plating liquid and is electrolytically treated, zinc ions are selectively deposited on the steel strip surface but it is difficult to deposit the rust resistant pigment fine particles into the zinc-plating layer. Therefore, it is very difficult to obtain a plated composite steel strip having an enhanced corrosion resistance.
Japanese Unexamined Patent Publication No. 60-211095 discloses a plated composite steel strip having a Zn-Ni alloy plating layer in which fine solid particles of metallic chromium, alumina (Al.sub.2 O.sub.3) or silica (SiO.sub.2) are co-deposited with and dispersed in a Zn-Ni alloy matrix. According to the disclosure of this Japanese publication No. 1095, the metallic chromium is obtained from chromium chloride (CrCl.sub.3). That is, chromium chloride is dissolved in the plating liquid and releases chromium ions (Cr.sup.3+). When the steel strip is immersed and electrolytically plated as a cathode in the plating liquid, metallic chromium particles and chromium oxide (Cr.sub.2 O.sub.3.nH.sub.2 O) particles are deposited into the playing layer to form a Zn-Ni alloy plating layer containing metallic chromium (Cr) and chromium oxide (Cr.sub.2 O.sub.3.nH.sub.2 O) particles.
When alumina or silica particles are further co-deposited into the Zn-Ni-Cr-Cr.sub.2 O.sub.3.nH.sub.2 O plating layer, the resultant plated composite steel strip exhibits an enhanced corrosion resistance compared with the plated composite steel having the Zn-Ni-Cr-Cr.sub.2 O.sub.3.nH.sub.2 O layer. However, the degree of enhancement of the corrosion resistance is small, and the Al.sub.2 O.sub.3 or SiO.sub.2 particle-containing, plated composite steel strip cannot realize a perforation resistance for at least 10 years.
Under the above-mentioned circumstances, it is desired by industry, especially the car industry, that a high corrosion resistance plated composite steel strip having a rust resistance for at least 5 years and a perforation resistance for at least 10 years be provided, and a method for producing the same.